1. Field of the Invention
This invention relates to an analog to digital conversion device and method. Specifically, the invention relates to an integrating and sampling and hold circuit for use in digitizing analog signals generated by a spectrophotometer or similar instrument.
2.Description of the Prior Art
Scientific instruments, such as spectrophctometers used for liquid chromatography, typically operate in several different speed modes. A typical spectrophotometer (see FIG. 1) operates in both a high resolution, low speed mode and a low resolution, high speed mode. In both modes, incident light 10 from a broadband light source 11 strikes a rotatable diffraction grating 16. Monochromatic light 17 from the grating 16 then passes through a beam splitter 19 which directs some portion of the light to a reference photodiode 21, and the remaining portion of the light through a sample cell 12 holding a fluid to be analyzed. In the high resolution mode, the rotating optical diffraction grating 16 is brought to a stop in its rotation prior to using the grating 16 to acquire data by photodiode 20. Typically, it takes about 20 miliseconds to rotate the grating 16 to the desired position, and about 20 miliseconds to acquire data in that position. Typically the high resolution mode provides about 24 data points per second. If the user of the instrument is interested in taking measurements at four different wave lengths, wherein each wave length is provided by one position of the grating 16, it is thus possible to take six data points per second on each wave length. In the high speed mode, the diffraction grating 16 is rotated continuously and not stopped at particular positions, and the data is acquired while the grating 16 is rotating. These high speed scans in the high speed mode can be performed repetitively, thus giving the user of the instrument a set of data representing continuous spectra. In the high speed mode, typically about 96 data points per second are taken.
The operation of this instrument presents a disadvantage in the high speed mode in that a large source of noise is introduced into the signal. Light 18 which is diffracted from the grating 16 impinges in the instrument typically on two conventional photodiodes. One photodiode 20 is for light passing through a sample cell 12 which holds the fluid to be analyzed, and the second photodiode 21 is a reference photodiode. The diffracted light 18 creates current in the photodiodes proportional to the light level striking each diode. The current from photodiode 20 is converted to a voltage by means of a conventional current to voltage converter circuit including an operational amplifier 22 and resistor R1. The voltage so produced is then converted to a signal varying in frequency by means of a conventional voltage to frequency (i.e., V/F) converter 24. The number of pulses in the frequency output in a known period of time is counted by a V/F counter 26. The known period of time is determined by an oscillator 25, providing typically a 16 MHz signal , and a counter 28. The V/F converter 24, oscillator 25, counters 26, 28, and controller 30, comprise a conventional ratiometric analog to digital converter. This circuit provides a level of intensity of the light 18 measured digitally.
In order to reduce error when the voltage to frequency converter 24 is operating at low frequencies, the high frequency counter 28 and the V/F counter 26 typically are started and stopped in synchronization with the voltage to frequency pulses output by the voltage to frequency converter 24. A microprocessor 30, which is typically provided in the spectrophotometer, gives a command for an analog to digital conversion operation to take place. At this point, the circuit waits for the next output pulse from the voltage to frequency converter 24, whereupon both the voltage to frequency converter 24 and the high frequency counter 28 start counting down from a preset value. When the high frequency counter 28 has counted down to zero, it resets to its original value, and the two counters 26, 28 then await the next voltage to frequency pulse output by the voltage to frequency converter 24, whereupon both counter 26 and counter 28 are stopped. The number of V/F pulses and high frequency pulses are then determined. The ratio of the number of V/F pulses to the number of high frequency pulses provides a number which is proportional to the amount of light impinging on a photodiode.
With the above-described analog to digital conversion process, the time of data acquisition can vary greatly in terms of both the starting time and ending time. In essence the starting time and ending time are defined by the time spent waiting for the next voltage to frequency pulse output by the voltage to frequency converter 24. This variance, or jitter, in the time of data acquisition disadvantageously creates inaccuracies when the diffraction grating 16 is rotating rapidly as it does in the high speed mode because there is a large background absorbance which varies significantly as a function of spectrophotometer wavelength, or grating position. This inaccuracy manifests itself as noise, as the background absorbance shifts from one scan to the next. In the high resolution mode, this is usually not a problem, because the grating is held stationary during the entire A/D process, including the jitter.